In air-conditioning apparatuses such as a multi-air-conditioning apparatus for a building, there is an air-conditioning apparatus that circulating a refrigerant from an outdoor unit to a relay unit and by circulating the heat medium such as water from the relay unit to the indoor units, thereby achieving a cooling and heating mixed operation while reducing the conveyance power of the heat medium (see Patent Literature 1, for example).
There is an air-conditioning apparatus that is equipped with a circuit that carries out liquid injection into a compressor through a high-pressure liquid pipe of a refrigeration cycle in order to reduce the discharge temperature of the compressor, in which the air-conditioning apparatus is capable of controlling the discharge temperature so as to accomplish a preset temperature regardless of the operation state (see Patent Literature 2, for example).
There is also an air-conditioning apparatus that carries out injection into a compressor (high-pressure shell compressor), whose hermetic vessel has a discharge pressure atmosphere therein, from an outlet side of a gas-liquid separator that is disposed in a high-pressure liquid pipe of a refrigeration cycle (see Patent Literature 3, for example).